1. Field of the Invention
This invention relates to coat protein (CP) genes derived from WT strains of cucumber mosaic virus (CMV). More specifically, the invention relates to the genetic engineering of plants and to a method for conferring viral resistance to a plant using an expression cassette encoding CP genes of WT strains of CMV.
2. Description of the Prior Art
Many agriculturally important crops are susceptible to infection by plant viruses, particularly CMV, which can seriously damage a crop, reduce its economic value to the grower, and increase its cost to the consumer. Attempts to control or prevent infection of a crop by a plant virus such as CMV have been made, yet viral pathogens continue to be a significant problem in agriculture.
Scientists have recently developed means to produce virus resistant plants using genetic engineering techniques. Such an approach is advantageous in that the genetic material which provides the protection is incorporated into the genome of the plant itself and can be passed on to its progeny. A host plant is resistant if it possesses the ability to suppress or retard the multiplication of a virus, or the development of pathogenic symptoms. “Resistant” is the opposite of “susceptible,” and may be divided into: (1) high, (2) moderate, or (3) low resistance, depending upon its effectiveness. Essentially, a resistant plant shows reduced or no symptom expression, and virus multiplication within it is reduced or negligible. Several different types of host resistance to viruses are recognized. The host may be resistant to: (1) establishment of infection, (2) virus multiplication, or (3) viral movement.
CMV is a single-stranded (+) ribonucleic acid (RNA) plant virus that has a functionally divided genome. The virus genome contains four RNA species designated RNAs 1-4. RNs 3 and 4 encode the coat protein (CP) which is a protein that surrounds the viral RNA and protects the viral RNA from being degraded. Only RNAs 1-3 are required for infectivity because the CP, which is encoded by RNA 4, is also encoded by RNA 3.
Several strains of CMV have been classified using serology, host range, peptide mapping, nucleic acid hybridization, and sequencing analyses. These CMV strains can be divided into two groups, which are designated “WT” (also known as subgroup I) and “S” (also known as subgroup II). The S group consists of at least three members. The WT group is known to contain at least 17 members.
Expression of the CP genes from tobacco mosaic virus, alfalfa mosaic virus, CMV, and potato virus X, among others, in transgenic plants has resulted in plants which are resistant to infection by the respective virus. Heterologous protection can also occur. For example, the expression of CP genes from watermelon mosaic virus-2 (WMV-2) or zucchini yellow mosaic virus (ZYMV) in transgenic tobacco plants has been shown to confer protection against six other potyviruses: bean yellow mosaic virus, potato virus Y, pea mosaic virus, clover yellow vein virus, pepper mottle virus, and tobacco etch virus. However, expression of a preselected CP gene does not reliably confer heterologous protection to a plant. For example, transgenic squash plants containing the CMV-C CP gene, a WT virus, which have been shown to be resistant to the CMV-C strain are not protected to the same degree against several other, highly virulent WT strains of CMV.
Thus, a need exists for plants resistant to WT strains of CMV.